Recently, solar cells are receiving public attention as a clean energy source of the next generation. In a solar cell module, a component such as a sealing sheet for a back surface which seals the back surface of the solar cell module is used and a film is used for the components thereof. Since a solar cell is used in outside for a long period of time, the components thereof and the film used for the components thereof are required to be durable under natural environment (such as light resistance and hydrolysis resistance).
As to the base film used in the sealing sheet for a back surface of a solar cell, there has been used a fluorine film, a polyethylene film or a polyester film to which a white pigment and the like are added, for the purpose of enhancing UV resistance, concealment properties and the like (Patent Documents 1 to 10).
Among these, Patent Document 10, which is disclosed by the applicant of the present invention, discloses a film having a multilayer structure in which a layer concentrically containing inorganic fine particles that contains inorganic fine particles in an amount of 10 to 35% by mass is disposed as at least one outermost layer to achieve a white polyester film for a solar cell that is excellent in environmental durability represented by light resistance and hydrolysis resistance while having high whiteness and good light reflectivity. It is described that titanium dioxide or barium sulfate, which is a white pigment, is preferably used as the above-mentioned inorganic fine particle from the view of the improvement of whiteness and productivity, and an example using titanium dioxide alone is described in the Example.
Furthermore, Patent Documents 11 and 12 disclose a white polyester film being suitably used for a reflector for a liquid crystal display and the like and having excellent luminance characteristics and the like.
Patent Document 11 discloses a white polyester film in which a skin layer (B) is provided on at least one surface of a polyester layer (A) containing air bubbles, and the skin layer (B) contains 5 to 30% inorganic particles (c) having a prescribed apparent density. It is described in Patent Document 11 that the polyester layer (A) containing air bubbles is required in terms of optical characteristics such as luminance and reflectance. In addition, it is described that the above-mentioned amount of the inorganic particles (c) is necessary for controlling the center plane average roughness (Ra) and the ten-point average roughness (Rz) of the film surface of the polyester (A) side to provide irregularities on the film surface, and hence the glossiness of the film can be lowered and luminance unevenness can be reduced.
Patent Document 12 discloses a white film having at least a layer A composed of polyester and a layer B composed of polyester, wherein the layer B has air bubbles, the layer A contains three types of inorganic particles of rutile type titanium oxide, barium sulfate and silicon dioxide. It is described that the layer B is whitened due to the presence of fine air bubbles in the film, and the formation of the fine air bubbles is achieved, for example, by allowing a polymer which is non-compatible with polyester to be finely dispersed and contained in polyester or allowing a polymer which is non-compatible with polyester and inorganic particles to be finely dispersed and contained in polyester, and stretching the resulting film. It is also described that in the layer A, by using the above-mentioned three types of inorganic particles in combination, anti UV performance, reduction in luminance unevenness, high reflectance, suppression of occurrence of flaws of a light guide plate and screen non-uniformity due to adhesion can be satisfied at the same time; and in particular, it is necessary to complementarily combine barium sulfate with rutile type titanium oxide, whereby good reflectance can be obtained, and luminance unevenness can be reduced. Furthermore, it is described that it is possible to allow a large number of fine air bubbles to be present even in the layer A since barium sulfate is non-compatible with polyester.